1. Field of the Invention
The present invention relates to a lens displacement device; in particular, to a displacement device applicable for moving micro image-capturing module lens.
2. Description of Related Art
In recent years, the trend of equipping hand-held devices with image-capturing modules is gradually becoming popular. While the product market of hand-held devices demonstrates stronger and stronger demands for better functionality and smaller size, the image-capturing module is at the same time confronted with two crucial requirements of higher resolution and size reduction. Quality enhancement of the captured images may be achieved by increasing pixels, whereby the image-capturing module in hand-held devices develops progressively from VGA-grade of 300K pixels to million, 2 millions, 3 millions or even higher to 4 or 5 millions pixels. In addition to the increase in pixel numbers, there are other factors such as image focusing that also affect image clarity. Thus the focusing function in the image-capturing module of hand-held device evolves from fixed focusing function to more advanced optical auto-focusing or even optically variable focusing as found in today's cameras.
The operational principle of optical auto-focusing function is to adequately move the lens in the image-capturing module according to different distance from the object appearing in the lens, such that the image of the intended object can precisely focus on the image sensor to generate clear image thereof. Currently, the common approaches of providing displacement functions to drive the motion of lens in an image-capturing module include step-motor displacement, piezoelectric displacement and voice coil based linear displacement etc. Herein the voice coil based linear displacement offers advantages of fast movement and direct linear motion, without the need of high precision screw rod transfer; meanwhile, at production stage, this approach can be more convenient, and allow flexibly combining integral space of image-capturing module for electro-magnetic field installation design, providing more favorable chance to reduce the integral size of image-capturing module. However, while emphasizing size-reduction, in order to acquire sufficient displacement force to move the lens, it also requires good design in magnetic circuits, so as to efficiently exploit limited magnetic force resource to generate sufficient displacement force.
Referring to FIG. 1, a structural diagram of a Voice Coil Motor (VCM) magnetic circuit in prior art is shown. The magnetic circuit part of a prior art VCM 1 is composed of two non-ring magnets 111, 112, two non-ring surface yoke blocks 121, 122 as well as a base yoke block 13 with a central ring wall, stacked up along the direction of VCM axle. The magnetic circuit of the prior art VCM 1 provides a closed magnetic circuit, which is used to generate magnetic flux 15. The magnetic flux 15 emerges from the upper surfaces of the two non-ring magnets 111, 112, through the two non-ring surface yoke blocks 121, 122, turns to inner ring gap 14, and reaches the central ring wall of the base yoke block 13, then further with guidance of the base yoke block 13, returns to the lower surfaces of the two non-ring magnets 111, 112. The design of this type of closed magnetic circuit uses two non-ring surface yoke blocks 121, 122 as guides of the magnetic flux 15 emitted from the two non-ring magnets 111, 112, obtaining the effect of magnetic flux 15 concentration, so as to generate sufficient displacement force.
The aforementioned VCM 1 is required to use yoke blocks for guiding the magnetic flux 15 to penetrate the coil (not shown) for generating displacement effect; however, the installation of yoke blocks increases the difficulty in manufacture. Meanwhile, it also restricts the installation of other components, causing bottleneck on size-reduction attempt.